2,831 research outputs found
Off-diagonal disorder in the Anderson model of localization
We examine the localization properties of the Anderson Hamiltonian with
additional off-diagonal disorder using the transfer-matrix method and
finite-size scaling. We compute the localization lengths and study the
metal-insulator transition (MIT) as a function of diagonal disorder, as well as
its energy dependence. Furthermore we investigate the different influence of
odd and even system sizes on the localization properties in quasi
one-dimensional systems. Applying the finite-size scaling approach in
conjunction with a nonlinear fitting procedure yields the critical parameters
of the MIT. In three dimensions, we find that the resulting critical exponent
of the localization length agrees with the exponent for the Anderson model with
pure diagonal disorder.Comment: 12 pages including 4 EPS figures, accepted for publication in phys.
stat. sol. (b
Glass phases of flux lattices in layered superconductors
We study a flux lattice which is parallel to superconducting layers, allowing
for dislocations and for disorder of both short wavelength and long wavelength.
We find that the long wavelength disorder has a significant effect on the phase
diagram -- it produces a first order transition within the Bragg glass phase
and leads to melting at strong disorder. This then allows a Friedel scenario of
2D superconductivity.Comment: 5 pages, 1 eps figure, Revte
Exciton states in monolayer MoSe2 and MoTe2 probed by upconversion spectroscopy
Transitions metal dichalcogenides (TMDs) are direct semiconductors in the
atomic monolayer (ML) limit with fascinating optical and spin-valley
properties. The strong optical absorption of up to 20 % for a single ML is
governed by excitons, electron-hole pairs bound by Coulomb attraction. Excited
exciton states in MoSe and MoTe monolayers have so far been elusive due
to their low oscillator strength and strong inhomogeneous broadening. Here we
show that encapsulation in hexagonal boron nitride results in emission line
width of the A:1 exciton below 1.5 meV and 3 meV in our MoSe and
MoTe monolayer samples, respectively. This allows us to investigate the
excited exciton states by photoluminescence upconversion spectroscopy for both
monolayer materials. The excitation laser is tuned into resonance with the
A:1 transition and we observe emission of excited exciton states up to 200
meV above the laser energy. We demonstrate bias control of the efficiency of
this non-linear optical process. At the origin of upconversion our model
calculations suggest an exciton-exciton (Auger) scattering mechanism specific
to TMD MLs involving an excited conduction band thus generating high energy
excitons with small wave-vectors. The optical transitions are further
investigated by white light reflectivity, photoluminescence excitation and
resonant Raman scattering confirming their origin as excited excitonic states
in monolayer thin semiconductors.Comment: 14 pages, 7 figures, main text and appendi
Representation of a complex Green function on a real basis: I. General Theory
When the Hamiltonian of a system is represented by a finite matrix,
constructed from a discrete basis, the matrix representation of the resolvent
covers only one branch. We show how all branches can be specified by the phase
of a complex unit of time. This permits the Hamiltonian matrix to be
constructed on a real basis; the only duty of the basis is to span the
dynamical region of space, without regard for the particular asymptotic
boundary conditions that pertain to the problem of interest.Comment: about 40 pages with 5 eps-figure
Approximating a Wavefunction as an Unconstrained Sum of Slater Determinants
The wavefunction for the multiparticle Schr\"odinger equation is a function
of many variables and satisfies an antisymmetry condition, so it is natural to
approximate it as a sum of Slater determinants. Many current methods do so, but
they impose additional structural constraints on the determinants, such as
orthogonality between orbitals or an excitation pattern. We present a method
without any such constraints, by which we hope to obtain much more efficient
expansions, and insight into the inherent structure of the wavefunction. We use
an integral formulation of the problem, a Green's function iteration, and a
fitting procedure based on the computational paradigm of separated
representations. The core procedure is the construction and solution of a
matrix-integral system derived from antisymmetric inner products involving the
potential operators. We show how to construct and solve this system with
computational complexity competitive with current methods.Comment: 30 page
Origin of four-fold anisotropy in square lattices of circular ferromagnetic dots
We discuss the four-fold anisotropy of in-plane ferromagnetic resonance (FMR)
field , found in a square lattice of circular Permalloy dots when the
interdot distance gets comparable to the dot diameter . The minimum
, along the lattice axes,
differ by 50 Oe at = 1.1. This anisotropy, not expected in
uniformly magnetized dots, is explained by a non-uniform magnetization
\bm(\br) in a dot in response to dipolar forces in the patterned magnetic
structure. It is well described by an iterative solution of a continuous
variational procedure.Comment: 4 pages, 3 figures, revtex, details of analytic calculation and new
references are adde
Relaxation of Spin Polarized He in Mixtures of He and He Below the He Lambda Point
We report the first study of the depolarization behavior of spin polarized
3He in a mixture of 3He-4He at a temperature below the 4He Lambda point in a
deuterated TetraPhenyl Butadiene-doped deuterated PolyStyrene (dTPB-dPS) coated
acrylic cell. In our experiment the measured 3He relaxation time is due to the
convolution of the 3He longitudinal relaxation time, T1, and the diffusion time
constant of 3He in superfluid 4He since depolarization takes place on the
walls. We have obtained a 3He relaxation time ~3000 seconds at a temperature
around 1.9K. We have shown that it's possible to achieve values of wall
depolarization probability on the order of (1-2)x10^-7 for polarized 3He in the
superfluid 4He from a dTPB-dPS coated acrylic surface.Comment: The Model used to interpret the data has been change
Use and Abuse of the Fisher Information Matrix in the Assessment of Gravitational-Wave Parameter-Estimation Prospects
The Fisher-matrix formalism is used routinely in the literature on
gravitational-wave detection to characterize the parameter-estimation
performance of gravitational-wave measurements, given parametrized models of
the waveforms, and assuming detector noise of known colored Gaussian
distribution. Unfortunately, the Fisher matrix can be a poor predictor of the
amount of information obtained from typical observations, especially for
waveforms with several parameters and relatively low expected signal-to-noise
ratios (SNR), or for waveforms depending weakly on one or more parameters, when
their priors are not taken into proper consideration. In this paper I discuss
these pitfalls; show how they occur, even for relatively strong signals, with a
commonly used template family for binary-inspiral waveforms; and describe
practical recipes to recognize them and cope with them.
Specifically, I answer the following questions: (i) What is the significance
of (quasi-)singular Fisher matrices, and how must we deal with them? (ii) When
is it necessary to take into account prior probability distributions for the
source parameters? (iii) When is the signal-to-noise ratio high enough to
believe the Fisher-matrix result? In addition, I provide general expressions
for the higher-order, beyond--Fisher-matrix terms in the 1/SNR expansions for
the expected parameter accuracies.Comment: 24 pages, 3 figures, previously known as "A User Manual for the
Fisher Information Matrix"; final, corrected PRD versio
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